Mechanical seal assemblies comprising stationary and rotating seal rings having faces opposed to one another are well known in the art. Generally, the stationary ring is connected to a housing and the rotating ring is connected to a rotatable shaft. The faces are lapped to the necessary flatness prior to assembly.
In many of the known seal assemblies, a back-up or compressing ring is located behind the rotating seal ring, the compression ring being connected to the rotating seal ring by a drive ring and one or more drive pins or keys, the keys being located in milled slots in the seal ring and in axially aligned slots in the back-up or compression ring. Ideally, the lapped faces of the seal rings are parallel to one another so that when leakage occurs between the faces, the pressure gradient produced acts as a separating force offsetting the closing force produced by the housing pressure acting on one seal face. Where high pressures are encountered, it is not always possible to keep the lapped surfaces flat. In some arrangements, the back-up ring has an outer diameter at its end adjacent to the seal ring which is approximately equal to that of the seal ring. The outer diameter of the back-up ring is larger at its other end and it has a cavity to receive a U-cup and the like. Because of the pressures on the seal ring and the back-up ring, the back-up ring distorts causing distortion of the seal ring and its lapped face. The lapped face becomes concave, thus providing a divergent seal leak path with a material reduction in seal leakage and a high unit load at a peripheral line of contact, conditions which are undesirable. A large amount of heat will be generated in the small contact surfaces involved and with reduced leakage, there is less heat being transferred from the seal faces, causing warpage of the ring leading to seal leakage. This will cause a sudden chilling of the lapped surfaces and heat checking of one or both of the seal rings.